Large-Area Fabrication of Hexaazatrinaphthylene-Based 2D Metal-Organic Framework Films for Flexible Photodetectors and Optoelectronic Synapses.

2D conjugated metal-organic frameworks (c-MOFs) have emerged as promising materials for (opto)electronic applications due to their excellent charge transport properties originating from the unique layered-stacked structures with extended in-plane conjugation. The further advancement of MOF-based (opto)electronics necessitates the development of novel 2D c-MOF thin films with high quality. Cu-HHHATN (HHHATN: hexahydroxyl-hexaazatrinaphthylene) is a recently reported 2D c-MOF featuring high in-plane conjugation, strong interlayer π-π stacking, and multiple coordination sites, while the production of its thin-film form has not yet been reported. Herein, large-area Cu-HHHATN thin films with preferential orientation, high uniformity, and smooth surfaces are realized by using a convenient layer-by-layer growth method. Flexible photodetectors are fabricated, showing broadband photoresponse ranging from UV to short-wave infrared (370 to 1450 nm). The relatively long relaxation time of photocurrent, which arises from the trapping of photocarriers, renders the device's synaptic plasticity similar to that of biological synapses, promising its use in neuromorphic visual systems. This work demonstrates the great potential of Cu-HHHATN thin films in flexible optoelectronic devices for various applications.

2D metal-organic frameworks for ultraflexible electrochemical transistors with high transconductance and fast response speeds.

Electrochemical transistors (ECTs) have shown broad applications in bioelectronics and neuromorphic devices due to their high transconductance, low working voltage, and versatile device design. To further improve the device performance, semiconductor materials with both high carrier mobilities and large capacitances in electrolytes are needed. Here, we demonstrate ECTs based on highly oriented two-dimensional conjugated metal-organic frameworks (2D c-MOFs). The ion-conductive vertical nanopores formed within the 2D c-MOFs films lead to the most convenient ion transfer in the bulk and high volumetric capacitance, endowing the devices with fast speeds and ultrahigh transconductance. Ultraflexible device arrays are successfully used for wearable on-skin recording of electrocardiogram (ECG) signals along different directions, which can provide various waveforms comparable with those of multilead ECG measurement systems for monitoring heart conditions. These results indicate that 2D c-MOFs are excellent semiconductor materials for high-performance ECTs with promising applications in flexible and wearable electronics.

Perovskite Solar Cell-Gated Organic Electrochemical Transistors for Flexible Photodetectors with Ultrahigh Sensitivity and Fast Response.

Photodetectors (PDs) are the building block of various imaging and sensing applications. However, commercially available PDs based on crystalline inorganic semiconductors cannot meet the requirements of emerging wearable/implantable applications due to their rigidity and fragility, which creates the need for flexible devices. Here, a high-performance flexible PD is presented by gating an organic electrochemical transistor (OECT) with a perovskite solar cell. Due to the ultrahigh transconductance of the OECT, the device demonstrates a high gain of ≈106 , a fast response time of 67 µs and an ultrahigh detectivity of 6.7 × 1017 Jones to light signals under a low working voltage (≤0.6 V). Thanks to the ultrahigh sensitivity and fast response, the device can track photoplethysmogram signals and peripheral oxygen saturation under ambient light and even provide contactless remote sensing, offering a low-power and convenient way for continuous vital signs monitoring. This work offers a novel strategy for realizing high-performance flexible PDs that are promising for low-power, user-friendly and wearable optoelectronics.

Highly Sensitive Broadband Phototransistors Based on Gradient Tin/Lead Mixed Perovskites.

Highly sensitive broadband photodetectors are critical to numerous cutting-edge technologies such as biomedical imaging, environment monitoring, and night vision. Here, phototransistors based on mixed Sn/Pb perovskites are reported, which demonstrate ultrahigh responsivity, gain and specific detectivity in a broadband from ultraviolet to near-infrared region. The interface properties of the perovskite phototransistors are optimized by a special three-step cleaning-healing-cleaning treatment, leading to a high hole mobility in the channel. The highly sensitive performance of the mixed Sn/Pb perovskite phototransistors can be attributed to the vertical compositional heterojunction automatically formed during the film deposition, which is helpful for the separation of photocarriers thereby enhancing a photogating effect in the perovskite channel. This work demonstrates a convenient approach to achieving high-performance phototransistors through tuning compositional gradient in mixed-metal perovskite channels.

2D Metal-Organic Framework Cu3 (HHTT)2 Films for Broadband Photodetectors from Ultraviolet to Mid-Infrared.

Cu3 (HHTT)2 (HHTT: 2,3,7,8,12,13-hexahydroxytetraazanaphthotetraphene) is a novel 2D conjugated metal-organic framework (2D c-MOF) with efficient in-plane d-π conjugations and strong interlayer π-π interactions while the growth of Cu3 (HHTT)2 thin films has never been reported until now. Here, the successful fabrication of highly oriented wafer-scale Cu3 (HHTT)2 thin films with a layer-by-layer growth method on various substrates is presented. Its semiconducting behavior and carrier transport mechanisms are clarified through temperature and frequency-dependent conductivity measurements. Flexible photodetectors based on Cu3 (HHTT)2 thin films exhibit reliable photoresponses at room temperature in a wavelength region from UV to mid-IR, which is much broader than those of solution-processed broadband photodetectors reported previously. Moreover, the photodetectors can show a typical synaptic behavior and excellent data recognition accuracy in artificial neural networks. This work opens a window for the exploration of high-performance and multifunctional optoelectronic devices based on 2D c-MOFs.

High-Performance Tin-Lead Mixed-Perovskite Solar Cells with Vertical Compositional Gradient.

Sn-Pb mixed perovskites with bandgaps in the range of 1.1-1.4 eV are ideal candidates for single-junction solar cells to approach the Shockley-Queisser limit. However, the efficiency and stability of Sn-Pb mixed-perovskite solar cells (PSCs) still lag far behind those of Pb-based counterparts due to the easy oxidation of Sn2+ . Here, a reducing agent 4-hydrazinobenzoic acid is introduced as an additive along with SnF2 to suppress the oxidation of Sn2+ . Meanwhile, a vertical Pb/Sn compositional gradient is formed spontaneously after an antisolvent treatment due to different solubility and crystallization kinetics of Sn- and Pb-based perovskites and it can be finely tuned by controlling the antisolvent temperature. Because the band structure of a perovskite is dependent on its composition, graded vertical heterojunctions are constructed in the perovskite films with a compositional gradient, which can enhance photocarrier separation and suppress carrier recombination in the resultant PSCs. Under optimal fabrication conditions, the Sn-Pb mixed PSCs show power conversion efficiency up to 22% along with excellent stability during light soaking.

2D materials for conducting holes from grain boundaries in perovskite solar cells.

Grain boundaries in organic-inorganic halide perovskite solar cells (PSCs) have been found to be detrimental to the photovoltaic performance of devices. Here, we develop a unique approach to overcome this problem by modifying the edges of perovskite grain boundaries with flakes of high-mobility two-dimensional (2D) materials via a convenient solution process. A synergistic effect between the 2D flakes and perovskite grain boundaries is observed for the first time, which can significantly enhance the performance of PSCs. We find that the 2D flakes can conduct holes from the grain boundaries to the hole transport layers in PSCs, thereby making hole channels in the grain boundaries of the devices. Hence, 2D flakes with high carrier mobilities and short distances to grain boundaries can induce a more pronounced performance enhancement of the devices. This work presents a cost-effective strategy for improving the performance of PSCs by using high-mobility 2D materials.

Gradient 2D/3D Perovskite Films Prepared by Hot-Casting for Sensitive Photodetectors.

2D Ruddlesden-Popper perovskites have attracted wide attention recently because of tunable optoelectronic properties and have been used as alternatives to their 3D counterparts in various optoelectronic devices. Here, a series of (PEA)2(MA) n -1Pb n I3 n +1 perovskite thin films is designed and fabricated by a convenient hot-casting method to obtain gradient n in the films, which leads to the formation of vertical heterojunctions that can enhance charge separation in the films under light illumination. Based on a single gradient perovskite film, a highly sensitive and stable photodetector with a responsivity up to 149 AW-1 and a specific detectivity of 2 × 1012 Jones is obtained. This work paves a way to realizing high-performance optoelectronic devices with enhanced charge separation by introducing compositional gradient in a perovskite film.

Lead-Free Perovskite/Organic Semiconductor Vertical Heterojunction for Highly Sensitive Photodetectors.

In recent years, photodetectors based on organic-inorganic lead halide perovskites have been studied extensively. However, the inclusion of lead in those materials can cause severe human health and environmental problems, which is undesirable for practical applications. Here, we report high-performance photodetectors with a tin-based perovskite/PEDOT:PSS vertical heterojunction. The device demonstrates a broadband photoresponse from NIR to UV. The maximum responsivity and gain are up to 2.6 × 106 A/W and 4.7 × 106, respectively. Moreover, a much shorter response time and higher detectivity can be achieved by reducing the thickness of PEDOT:PSS. The outstanding performance is due to the excellent optoelectronic properties of the perovskite and the photogating effect originating from the heterojunction. Furthermore, devices fabricated on flexible substrates can demonstrate not only high sensitivity but also excellent bending stability. This work opens up the opportunity of using lead-free perovskite in highly sensitive photodetectors with vertical heterojunctions.

A General Wet Transferring Approach for Diffusion-Facilitated Space-Confined Grown Perovskite Single-Crystalline Optoelectronic Thin Films.

Hybrid perovskite single-crystalline thin films are promising for making high-performance perovskite optoelectronic devices due to their superior physical properties. However, it is still challenging to incorporate them into multilayer devices because of their on-substrate growth. Here, a wet transfer method is used in transferring perovskite single-crystalline films perfectly onto various target substrates. More importantly, large millimeter-scaled single-crystalline films can be obtained via a diffusion-facilitated space-confined growth method as thin as a few hundred nanometers, which are capable of sustaining excellent crystalline quality and morphology after the transferring process. The availability of these crystalline films offers us a convenient route to further investigate their intrinsic properties of hybrid perovskites. We also demonstrate that the wet transfer method can be used for scalable fabrication of perovskite single-crystalline film-based photodetectors exhibiting a remarkable photoresponsivity. It is expected that this transferring strategy would promise broad applications of perovskite single-crystalline films for more complex perovskite devices.

Sn-Based Perovskite for Highly Sensitive Photodetectors.

Organic-inorganic hybrid perovskites have emerged as promising functional materials for high-performance photodetectors. However, the toxicity of Pb and the lack of internal gain mechanism in typical perovskites significantly hinder their practical applications. Herein, a low-voltage and high-performance photodetector based on a single layer of lead-free Sn-based perovskite film is reported. The device shows broadband response from ultraviolet to near-infrared light with a responsivity up to 105 A W-1 and a high gain at a low operating voltage. The outstanding performance is attributed to the high hole mobility, p-doping nature, and excellent optoelectronic properties of the Sn-based perovskite. Moreover, the device is assembled on a flexible substrate and demonstrates both high sensitivity and good bending stability. This work demonstrates a route for realizing nontoxic, low-cost, and high-performance perovskite photodetectors with a simple device structure.

Antioxidant Grain Passivation for Air-Stable Tin-Based Perovskite Solar Cells.

Tin-based perovskites with excellent optoelectronic properties and suitable band gaps are promising candidates for the preparation of efficient lead-free perovskite solar cells (PSCs). However, it is challenging to prepare highly stable and efficient tin-based PSCs because Sn2+ in perovskites can be easily oxidized to Sn4+ upon air exposure. Here we report the fabrication of air-stable FASnI3 solar cells by introducing hydroxybenzene sulfonic acid or its salt as an antioxidant additive into the perovskite precursor solution along with excess SnCl2 . The interaction between the sulfonate group and the Sn2+ ion enables the in situ encapsulation of the perovskite grains with a SnCl2 -additive complex layer, which results in greatly enhanced oxidation stability of the perovskite film. The corresponding PSCs are able to maintain 80 % of the efficiency over 500 h upon air exposure without encapsulation, which is over ten times longer than the best result reported previously. Our results suggest a possible strategy for the future design of efficient and stable tin-based PSCs.